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### DELAWARE MOSQUITO CONTROL SECTION



## SPRAY THRESHOLD CRITERIA

General Comments

What is Involved in Making Spray Decisions -- Determination of when and where mosquito control insecticide spraying might be needed, whether for larvicides or adulticides, is based upo n extensive and intensive surveillance-and-monitoring efforts for immature or adult mosquito populations using several types of indi cators for larval or adult mosquitoes, as well as sometimes th e presence in the environment of mosquitotransmitted viruses of special concern to hum an health (e.g. eastern equine encephalitis, West Nile virus). No one factor is always decisive for when or where to spray, and often a combination of quantitative or objective da ta must be evaluated to make “spray/nospray” decisions in quasi-s ubjective manner (including also factoring in current or predicted future conditions for weather or ti des that affect win dows-of-opportunity for when spraying might best be done).

What Does It Mean to Exceed Threshold Values -- A long history in Delaware (>45 years) of our “real world” experiences in developing and using the threshold criteria

below indicates that when a criterion or co mbination of criteria are exceeded and we do not initiate some control actions by larvic iding or adulticiding as appropriate, then mosquito infestations of a level unacceptabl e to the public’s quality-of-life, comfort and well-being, and often detrimen tal to local economies too, will either soon ensue or continue unabated. Additionally, if these infe stations are not satisfactorily prevented or controlled, there can then often be corollary substantive concerns a bout potential public health problems in the form of mosquito-borne diseases. Therefore, the numeric values for the thresholds provided below are alwa ys subject to further refinement or modifications by the Mosquito Control Sect ion as circumstances or new information warrants, in that these criteria have evol ved over time and will probably continue to do

so.

What Mosquitoes Need Control -- By no means do all mosquitoes in Delaware need control, since of the approximate 57 mosquito species found in the state, only about 19 species are pestiferous or problematic to humans or domestic animals. Factors in determining how pestiferous a mosquito spec ies might become include how abundant or numerous can the adult populations of these species become, and what are their flight ranges, host preferences, biting aggressiveness , or potential for transmitting diseases. Contending with one or more of these 19 pest iferous species results in mosquito control spraying in Delaware that routinely star ts about mid-March with larviciding for woodland-pool-breeding species, then progresses to salt marsh larviciding starting as early as April in some years, with adulticiding of saltmarsh mosquitoes sometimes also needed as early as mid-April, and then con tinuing with various t ypes of spraying around the state throughout the remai nder of the control season up until as late as early November, including efforts to suppress contai ner-breeding species in urban or domestic settings, and the control of mosquito pr oduction in myriad freshwater habitats.2 Who Determines the Threshold Criteria -- By State statute, the Mosquito Control Section has the responsibility and authority for determini ng and using these threshold criteria for making decisions about when and where mosquito control spraying is needed for any-and-all lands in Delaware, whether the lands be federal, state, county, municipal or private properties. This is not necessar ily one-in-the-same as determining when and where spraying will actually be allowed to be done (e.g. on federal National Wildlife Refuges, the U.S. Fish and Wildlife Servi ce exercises final aut hority over whether any spraying will be allowed), or will even be acceptable for any given landowner. Nonetheless, in order to effectively treat public nuisances or prevent disease, the Mosquito Control Section has the statutor y authority to enter upon all lands under the State’s jurisdiction including pr ivate lands, and take whatever control actions are needed, including spraying without la ndowner permission or concurrence, doing such while avoiding any unnecessary damage; but for a ny source reduction projects involving habitat modifications, we try not to exercise such authority without coope ration or consent of

landowners.

Threshold Criteria for Application of Larvicides

Wetland or “Pothole” Larval Dipper Counts (applicable to saltmarsh or freshwater

wetlands pestiferous species) – for larval dipper count s (using basically a coffee-cup-size container on a stick designed to take sha llow-water samples), where larvae can range

from 1 st to 4 th instars and also include pupae, our finding any larvae in >25% of all sampled sites that then averag e >5 larvae/dip (for all dips, including “zeros”) can warrant larviciding throughout the wetlands where th e sampling runs were performed. Thus, these threshold criteria incorporate minimum triggers for both the spatial extent or frequency of any breeding observed (>25% of sampled sites), and for the intensity or amount of any breeding observed (a verage >5 larvae/dip) – i.e. for any larviciding to occur in a sampled marsh or wetlands, breed ing should be observed in >25% of all the breeding sites sampled, AND be occurri ng at an average intensity of >5 larvae/dip (incorporating all dips into this average). Additionally, for saltmarsh mosquito breeding leading to larviciding effort s, an additional factor is added whereby >25% of the marsh surface in “tussocky” salt marshes, or >25% of pothole habitats in firmer saltmarshes, should be estimated to be inundated or holding wa ter, as a measure of the extent of potential breeding habitats actu ally holding water. [An estimate of the % pupae present in saltmarsh or freshwater breedin g habitats also gives us an indication as to whether it might be too late to attempt e ffective larvicide treatments.] Experience has

shown us that if we don’t take timely larviciding actions befo re a mosquito brood of such problematic scope or intensity emerges, then it will be a matter of only a few to several days before adulticiding will then become nece ssary in either nearby or even quite distant locations depending upon the t ypes of mosquito species involved, along with weather factors such air temperature or wind speed and direction.

Larval Sampling Stations -- Larval sampling stations ar e described in our larval sampling plans or given via larval inspection in structions from Secti on managers to field personnel, such that timely and good spatial representation of poten tial breeding sites is3 achieved, with the sampling sites selected having environmental characteristics known or thought to favorable for mosquito-breeding. By recent rainfall or tidal flooding patterns, we usually have a pretty good idea of the best timing to undertake our larval surveillance efforts to yield the best-return-for-effort. Refinements for the need to larvicide any geographic area or location where observed breeding exceeds threshold criteria can be made based upon the types of mosquito specie s found and their typical flight ranges, as well as a breeding site’s proximity to human population concentrations, sometimes resulting in our not taking larval control actions even though threshold criteria are exceeded. For example, long-distance-flying saltmarsh mosquitoes (often roaming 3-5 miles up to 10-15 miles from their natal marshes) will typically warrant larviciding anywhere the breeding threshold criteria ar e exceeded; whereas some relatively shortdistance-flying freshwater spec ies (flying from only a few hundred yards up to 1-2 miles from their natal habitats) might not, especially in areas remote from human populations.

Larval Identifications -- The levels to which larval identifications need to be taken for operational purposes can vary depending upon the environmental setting and the operational utility of making such taxonomic identifications. For example, in salt marshes all 5 saltmarsh mosquito species en countered in Delaware are known to be problematic for humans, so for salt marsh surveillance any-and-al l larvae can be of concern, and just counting larvae without re gard to specific species suffices for most operations, which is especially true for the 3 saltmarsh Ochlerotatus ( Aedes ) species that dominate open salt marsh mosquito producti on, and to a lesser extent for the single Culex and single Anopheles species found in saltmarshes. The saltmarsh Culex and Anopheles species are more commonly associated w ith impounded or more permanent-water breeding habitats in our co astal wetlands, and also don’t fly as far as the 3 Ochlerotatus ( Aedes ) species, so unless populated areas are within 1-2 miles from where saltmarsh

Culex or Anopheles larvae are detected, in most cas es it wouldn’t be necessary to larvicide impounded or more permanent water salt marsh habitats (that also lacked significant Ochlerotatus ( Aedes ) breeding). Fortunately, it is relatively easy to distinguish in the field among Ochlerotatus ( Aedes ) , Culex and Anopheles larvae at the genus level when doing dipper counts. In most freshwater environmen ts, particularly in wet woodland pools, meadow swales or roadsi de ditches, finding and identifying in the

field the larvae of Ochlerotatus, Aedes or Psorophora genera can indicate a problem regardless of species; and wh en populated areas are found w ithin 1-2 miles of such freshwater breeding habitats, so can findi ng and identifying in the field larvae of Culex or Anopheles genera regardless of sp ecies. Finding larvae of Ochlerotatus, Aedes, Psorophora, Culex or Anopheles genera regardless of species in or near populated areas, whether the larvae are found in standing wate r, ephemeral water, or container-breeding habitats (including for the latt er any man-made structures holding water for more than 4 consecutive days, or in natural treeholes), can all be problematic. Overall for operational purposes in almost all types of breeding habitats, identifying larvae in the field just to the genus level quite nicely suffices. Finally, it should be noted that finding larvae of Coquilletidia perturbans can also be indicative of a pr oblem situation, but because of their larval requirements for burying into se diments and attaching to rooted aquatic vegetation, these larvae are rare ly observed in the field without purposely sampling for4 them, and such larval habitat also does not lend itself to larviciding, so adulticiding becomes the primary form of control for this species.

“ Container” Occurrences (applicable to larvae of pestiferous species found in natural or

man-made containers or other water- holding structures) – any larvae of pestiferous species (genera) observed in or sampled fr om container-like habitats might warrant control, especially if close to human habitations and standi ng water will remain for more than 5 consecutive days. Primary species of concern will be those found in genera Culex

, Ochlerotatus or Aedes. If the “containers” lend themse lves to dipper sampling, then

larval counts averaging >2 larvae/dip might warrant control actio ns. If larval control cannot be achieved by source reduction (e.g. dumping or eliminating standing water), then larviciding might be needed, almost al ways done via ground- or hand-applications (exceptions involving aerial a pplications might occur for sa lvage yards, tire dumps, or sewage lagoons).

Threshold Criteria for Application of Adulticides

Landing Rate Counts (applicable to adult biting mosqu itoes landing on a field inspector during a one-minute period in or nearby human habitations or within flight ranges of human population concentrations, with the c ounts taken anytime between early morning through early evening) – in populated areas (such as cities, towns or suburbs, or in exurban subdivisions or larger strip-developments ) , landing rate count averages for biting mosquitoes of >2-3 mosquitoes/minute (translating into 60-90 or more mosquito bites in a half-hour in somebody’s b ackyard) can trigger the need for adulticiding; whereas in relatively unpopulated areas (often rural loca tions with low population densities, and commonly in or near wet woodlands or marshy staging areas), landing rate averages for biting mosquitoes of >5-10 mosquitoes/minute (becoming 150-300 or more mosquito bites in only a half-hour) might be needed to warrant adulticiding. [In some locations at certain times, landing rate counts of 50-100 mosquitoes/minute have been recorded!]

The results obtained with landi ng rate counts can be almost immediately diagnostic of a

need to adulticide.

How to Perform Landing Rate Counts -- The protocol for conducting landing rate counts consists of counting all mosquitoes obs erved landing on all readily visible parts of an inspector’s body in one minute intervals, with the inspector standing still and making very little movement, along with not using any t ype of repellent. At any particular site, specific standing locations for conducting the counts are selected fa voring shaded spots near vegetation having little or no wind. When it is obvious that a landed mosquito is either biting or about to bite an inspector, the inspector is allowed to undertake subtle movements during the count to terminate or prevent the bite (which makes for very interesting movements when dozens of mosquitoes might be simultaneously trying to bite!); and care is also taken to the extent possible not to count any single mosquito more than once during any one-minute count.

When to Take Landing Rate Counts -- Because of normal working hours for State employees, most landing rate counts are conduc ted during the daytime, which in allowing5 for commuting time then leads to most count s being conducted in the field between about 8:30 am and 4:00 pm. This can work well for saltmarsh and wet woodland Ochlerotatus ( Aedes ) spp., for freshwater Psorophora spp. and Aedes vexans , and for the containerbreeding Aedes albopictus , but often tends to underestimate the abundance on-wing that’ll be found in the early evening, throughout the night, or early in the morning for

Culex and Anopheles spp., and to a lesser extent for Coquilletidia perturbans. Whenever possible and practicable to do, particularly where pertinent for dealing with potential “hot” situations, landing rate counts are also conducted near or at dusk or in the early morning. But it can be safely assumed th at any mosquito landing upon an inspector is one of the 19 problematic species, and hence any-and-all mosquitoes observed alighting during a landing rate count are included in the tally.

New Jersey-style Adult Light Trap Counts (collected via unbaited NJ adult light traps at a few dozen “permanent” locations around the state, typically set within populated areas, or sometimes set in locations indica tive of potential problems for nearby populated areas) – having nightly NJ adult light trap counts containing >25 females/trap of pestiferous species, checked the following day after a night’s trapping period, or which average >25 females/trap of pestiferous species for multi-night collections, are indicative of adult mosquito populations on-wing in th e general vicinity that would have been intolerable to most people the night(s) before . Because of the lag time involved with collecting and analyzin g adult light trap collections, these counts often are not as immediately diagnostic of any need to adultici de as are landing rate counts; but within about 48-72 hours of problems arising from excessive numbers of adult pestiferous mosquitoes on-wing, the light trap counts can serve as in dicators of the need to adulticide, or can help document how effec tive an earlier control treatment might have been. However, even with such a delay in immediate operational utility, the light trap counts are still valuable for allowing us to examine many of the types of pestiferous mosquitoes that might be around, at least fo r the trap-susceptible species, allowing for year-to-year comparisons at any given locat ion and month-to-month comparisons within any given year.

CDC-style Portable Adult Light Trap Counts (collected via CO 2 -baited portable CDC adult light traps at temporary collection stations around the st ate, often set and tended in location types similar to where NJ adult light traps are deployed, and often set in response to other indicators of mosquito problems, par ticularly to determine or to verify “hot spots”) -- nightly CDC adult light trap c ounts collected the foll owing day containing >50 females/trap of pestiferous species are indicative of adult mo squito populations on-wing in the general vicinity that would have been intolerable to most people the night before. [The CO 2 -baited nature and other in herent design features of the CDC-style traps cause them to collect more mosquitoes per night than NJ-style adult light traps, but the NJ-style traps are more durable and less costly to opera te than the CDC-style traps, and the State of Delaware also has a much longer historic database using NJ-s tyle traps at wellestablished, fixed locations.] Because ofthe urgency with which these portable CDCstyle traps are often se t and tended to and then analyzed, they can be diagnostic of the

need to adulticide within about 24-36 hours of their setting.6 Public Complaints About Biting Mosquitoes – these are called into or otherwise communicated to either our upstate or downs tate Mosquito Control Section operational headquarters (in Glasgow for New Castle County and north western Kent County, and in Milford for the remainder of Kent County and Sussex County) by citizens or elected officials, and usually involve people wanting to report or complain about unacceptable or intolerable numbers of biting mosquitoes on-wing, followed by their then requesting some adulticiding (although sometimes the re ports involve concerns about observations of standing or stagnant wate r bodies that might produce mosquitoes, which they either want us to inspect and treat with larvicid es or to somehow eliminate). Receiving complaints from the public is an invaluable way to help us focus and make best use of our limited control resources – depend ing upon weather conditions and other environmental factors during any given year, on a statewide basis th e Mosquito Control Section typically receives from about 1500- 3000 public complaints about too many biting mosquitoes, conveyed to us from early April into early November.

Reactions to Public Complaints -- Depending upon patterns for the geographic locations, densities and intens ities of public complaints received, the Mosquito Control Section might then undertake some adulticide spraying. When practicable to do within limits of our staff availability and working re sources, before making any final decision to spray just based upon public requests, we often try to integrate thes e calls for treatment with other available mosquito-problem indicat ors, such as landing rate counts or adult light trap counts. Over the period of many years of our doing this, we have also come to know many individuals whose reque sts for some adulticiding relief to suppress mosquito populations are unfailingly accurate and repres entative of truly intolerable quality-of-life conditions caused by too many biting mosquitoes , and we tend to pay extra attention to many of these regular “trusty” callers. We also take quite serious those requests coming from city or town officials for our adulticidi ng services, for those m unicipalities that have endorsed our annual Spray Policy’s requirement s and protocols. And to repeat here, whenever possible and practicable to do, our responding to public complaints by actually spraying is also first coupled with other indicators of adult mosquito abundances, including landing rate counts or adult light trap counts pe rtinent to the areas where adulticiding requests have arisen.

Indicators of Mosquito-Borne Diseases

The Mosquito Control Section conducts surv eillance-and-monitoring for mosquito-borne

diseases of note to humans that in Delaware primarily concerns two arboviruses -- eastern equine encephalitis (EEE) and We st Nile virus (WNV).

Sentinel Chickens -- The Section operates a statewide network of a few dozen “sentinel chicken” monitoring stations each year, where by from about late May into late October or early November about one-half of our sent inel birds are tested every week for the presence of antibodies indicat ive that they have might c ontracted EEE or WNV, yielding for us a good signal of the viruses’ presence and transmission within the environment.

The sentinel chicken flock locations are select ed to give us good ge ographic coverage (or at least as much as we can afford) for genera l areas either known or suspect to have good7 virus potential. Each sentinel chicken stati on (or “flock”) consists of a total of 4 birds that are humanely housed and cared for in 2 wire mesh cages per station, with 2 birds each week from each station having small blood samples drawn for virus testing, thereby causing any individual chicken to be sample d but once every two weeks. The bleeding does not kill or harm the birds, and any ch ickens that might cont ract EEE or WNV also

do not die from these viruses.

Sick or Dead Wild Birds -- The Mosquito Control Secti on also operates from April through October a statewide network for collect ing and reporting sick or dead wild birds suspect to have WNV, relying primarily upon public reports of su spect wild birds, collection of good candidate birds by the Sect ion, and testing for the presence of WNV by the state’s Division of Public Health Laboratory. The Section has a statewide geographic strategy for accepting and testing w ild birds that helps to ensure good spatial coverage and timely monitoring.

Mosquito Collections -- Occasionally on an ad hoc basis, the Mosquito Control Section will also analyze mosquito collections for the presence of WNV or EEE, sometimes with an interest in the specific species of mosqu itoes carrying the viruses, or sometimes with only a more general interest in documenting th e presence of viruses at the genus or guild levels. Samples for such analyses are usually collected by portable adu lt light traps. This might be done in conjunction with or as follo w-up to other indicati ons of virus presence, particularly in known or su spect virus “hot spots.”

Horse/Human Cases -- The Mosquito Control Section also receives timely reports from the State Veterinarian for the finding of EEE or WNV in unvaccinated horses; and we

receive from the state’s Divi sion of Public Health timely reports of any EEE or WNV

human cases.

What Does Presence of Virus Mean? -- Indications of the pr esence of EEE or WNV by any of the methods above is then factored bot h spatially and tempora lly into the types and

extent of control actions that we take. In part we use such indications to try to increase the public’s awareness to take some persona l protection measures against being bitten, and for them to also practice good water sa nitation around their homes or businesses to reduce mosquito production. We also use such virus indications to increase to the extent possible and practicable our surveillance-and -monitoring actions for assessing mosquito populations, and to increase our mosquito co ntrol treatment efforts accordingly, since one of the more important factors in preventing mosquito-borne diseases is to reduce even further populations of bridge-v ector species, in order to de crease the probability even further that people will get bit by disease-carrying mosquito es (i.e. good nuisance control = good disease prevention or contro l). This increase in our c ontrol responses relative to findings of virus presence is typically achieved by our then reducing the spray threshold

criteria associated with indicators of mos quito abundance, in th at we then undertake spray actions at lower larval dipper counts, lower landing rate counts, lower adult light

trap counts, lesser numbers of public complaints, etc. than what have been indicated

above for when there are no indications of di sease. By how much we actually lower

these threshold criteria in response to virus presence is fairly subjective, based upon our8 semi-quantitative estimations of virus presence; but since human health is involved here,

we of course tend to-err-on-the-side-of-cauti on (within limits of our available treatment resources and applicable envi ronmental considerations).

Last revision – June, 2008